1. Field of the Invention
The present invention relates in general to an oxygen sensing apparatus adapted to detect an oxygen concentration of combustion exhaust gases such as those emitted by internal combustion engines of automobiles or various industrial furnaces, and more specifically to such an oxygen sensor used for a combustion control system for internal combustion engines and industrial furnaces, to determine an air/fuel (A/F) ratio of an air-fuel mixture supplied to the engines and furnaces.
2. Discussion of the Prior Art
Various types of oxygen sensors are known for determining an oxygen concentration of combustion exhaust gases emitted for example by automotive internal combustion engines, industrial furnaces or boilers. For instance, there is known a sensor which utilizes a zirconia ceramic or other oxygen-ion conductive solid electrolyte material and which is operated to determine the oxygen concentration according to the principle of an oxygen concentration cell. For operating an internal combustion engine, it is required to accurately control an air/fuel (A/F) ratio of an air-fuel mixture supplied to the engine, such that the actual air/fuel ratio coincides with a desired or nominal value. Generally, this air/fuel ratio is determined by measuring the concentration of oxygen in the exhaust gases, which is varied as a function of the air/fuel ratio of the air-fuel mixture supplied to the engine. A signal representative of the air/fuel ratio is fed to a fuel supply control system of the engine, in order to determine an amount of supply of the fuel, i.e., to control the fuel supply in a feedback manner so that the actual air/fuel ratio coincides with the desired value.
As one type of such an oxygen sensor used as an A/F-ratio sensor, there is known a so-called double-cell type sensor having two electrochemical cells, that is a pumping cell and a sensing cell, as disclosed in laid-open Publication No. 59-190652 of unexamined Japanese Patent Application. The A/F-ratio sensor of this type is capable of dealing with not only stoichiometric exhaust gases emitted by combustion of an air-fuel mixture whose A/F ratio is equal to or near the stoichiometric value (A/F=14.6), but also lean-burned or rich-burned exhaust gases emitted by combustion of a fuel-lean or fuel-rich air-fuel mixture whose A/F ratio is larger or smaller than the stoichiometric value. In this double-cell type A/F-ratio sensor, a sensing element of the sensor is formed with an internal gas-diffusion space into which a measurement gas (exhaust gases) is introduced from an external measurement-gas space under a predetermined diffusion resistance. The electrochemical oxygen sensing cell includes a measuring electrode exposed to the atmosphere within the internal gas-diffusion space, and a reference electrode exposed to a reference atmosphere which has a predetermined oxygen partial pressure. This sensing cell produces an output in the form of an electromotive force which is induced according to the principle of an oxygen concentration cell. The electrochemical oxygen pumping cell includes an outer pumping electrode exposed to the measurement gas existing in the external measurement-gas space, and an inner pumping electrode exposed to the atmosphere within the internal gas-diffusion space. This oxygen pumping cell is operated to effect a pumping action so as to control the atmosphere in the internal gas-diffusion space. The sensing element constructed as described above is adapted to measure the oxygen concentration of the measurement gas based on a pump current applied to the oxygen pumping cell to effect the pumping action so as to control the oxygen concentration of the atmosphere within the internal gas-diffusion space such that the electromotive force produced by the oxygen sensing cell coincides with a predetermined value.
If there arises a difference in the oxygen partial pressure between the atmosphere (within the gas-diffusion space) which contacts the measuring electrode of the oxygen sensing cell, and the atmosphere (within the gas-diffusion space) which contacts the inner pumping electrode of the oxygen pumping cell, the A/F-ratio sensor of the above type suffers from reduction in the operating response and deterioration of the oxygen pumping cell.
In view of the above, there is proposed a method of determining the concentration of a given component (such as oxygen) in the measurement gas, as disclosed in U.S. Pat. No. 4,645,572 (laid-open Publication No. 61-194345 of unexamined Japanese Patent Application). In the oxygen sensor disclosed therein, a first solid electrolyte body for the oxygen sensing cell and a second solid electrolyte body for the oxygen pumping cell are electrically connected to each other through a solid electrolyte layer interposed therebetween. Further, the measuring electrode of the sensing cell and one of the inner and outer electrodes of the pumping cell are connected to a reference conductor (e.g., the earth) which provides a reference potential for permitting a pump current to flow through the pumping cell. With the measuring and inner or outer pumping electrodes having the same potential, a portion of the pump current applied to the oxygen pumping cell leaks toward the measuring electrode of the oxygen sensing cell. In this arrangement, an auxiliary pumping action is effected at the measuring electrode of the oxygen sensing cell, by the leak current which leaks from the oxygen pumping cell. Consequently, the oxygen sensor of this type is capable of compensating the sensor output for the difference between the oxygen partial pressure of the atmosphere which contacts or surrounds the measuring electrode, and the atmosphere which contacts or surrounds the inner pumping electrode.
However, a further analysis by the present inventors of the oxygen sensor of the above type revealed that the leak current which flows from the oxygen pumping cell toward the oxygen sensing cell causes a potential change in the oxygen sensing cell due to a resistance potential drop, whereby the sensing accuracy of the oxygen sensor is lowered. In addition, since the amount of the potential change due to the resistance potential drop is varied with the internal resistance value of the solid electrolyte body of the sensing cell, the sensing accuracy of the sensor tends to be largely dependent on the temperature of the sensing cell.
Further, the auxiliary pumping action of the measuring electrode by the leak pump current causes a potential drop of the measuring electrode upon ionization of oxygen (O.sub.2). This potential drop changes the output characteristic of the oxygen sensing cell and lowers the sensing accuracy of the oxygen sensor. The amount of this potential drop of the measuring electrode due to the auxiliary pumping action is likely to be influenced by the surface condition of the measuring electrode or surface condition of a protecting layer covering the electrode, that is, influenced by a change in the gas diffusion resistance around the measuring electrode. More specifically, if the gas diffusion resistance is changed by gas absorption or deposition of foreign particles during use of the sensor, the output characteristic of the oxygen sensor is remarkably changed, making it difficult to assure sufficiently high chronological operating stability and reliability of the sensor.
The lowered operating accuracy of the sensor or the chronological change of the output characteristic of the sensor, which results from leaking of the pump current from the oxygen pumping cell toward the oxygen sensing cell, is particularly serious when the pump current is relatively high, that is, when the oxygen sensor as the A/F-ratio sensor is used to deal with lean-burned or rich-burned exhaust gases emitted by combustion of an air-fuel mixture whose A/F ratio is larger or smaller than the stoichiometric value. Thus, there remains room for improvement in the known oxygen sensor as described above.